x86 Instruction Set Reference

# SGDT

# Store Global Descriptor Table Register

Opcode Mnemonic Description
0F 01 /0 SGDT m Store GDTR to m.

# Description

Stores the content of the global descriptor table register (GDTR) in the destination operand. The destination operand specifies a 6-byte memory location. If the operand-size attribute is 32 bits, the 16-bit limit field of the register is stored in the low 2 bytes of the memory location and the 32-bit base address is stored in the high 4 bytes. If the operand-size attribute is 16 bits, the limit is stored in the low 2 bytes and the 24-bit base address is stored in the third, fourth, and fifth byte, with the sixth byte filled with 0s.

SGDT is only useful in operating-system software; however, it can be used in application programs without causing an exception to be generated.

See "LGDT/LIDT-Load Global/Interrupt Descriptor Table Register" in Chapter 3 for information on loading the GDTR and IDTR.

# Operation

if(OperandSize == 16) {
	Destination[0..15] = GDTR.Limit;
	Destination[16..39] = GDTR.Base; //24 bits of base address loaded
	Destination[40..47] = 0;
}
else { //32-bit Operand Size
	Destination[0..15] = GDTR.Limit;
	Destination[16..47] = GDTR.Base; //full 32-bit base address loaded
}
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# IA-32 Architecture Compatibility

The 16-bit form of the SGDT is compatible with the Intel 286 processor if the upper 8 bits are not referenced. The Intel 286 processor fills these bits with 1s; the Pentium 4, Intel Xeon, P6 family, Pentium, Intel486, and Intel386 processors fill these bits with 0s.

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